It is now easy for us to understand why repentance is always joined to Christian faith, and why the Lord asserts that no one can enter the kingdom of heaven without being born again (John 3:3).
Repentance is that turning round by which, leaving behind the perversity of this world, we come back on to the Lord’s path. And seeing that Christ is no minister of sin, if He cleanses us from the stains of sin and clothes us with participation in His righteousness, it is not so that we might then profane such great grace with new offences. It is, rather, that we might dedicate the future course of our life to the glory of the Father who has adopted us as His children.
The coming about of this repentance depends on our new birth and comprises two aspects: the putting to death of our flesh (that is to say, of our inborn corruption) and the spiritual coming to life by which human nature is restored to uprightness.
Throughout our life, then, we are to reflect on the fact that we are dead to sin and to ourselves, in order to live for Christ and His righteousness. And seeing that this process of rebirth is never completed as long as we are the prisoners of this mortal body, we should be careful to continue repenting until we die.
John Calvin, Truth For All Time, pp. 32, 33